Concentrated photovoltaic (CPV) systems operate by focusing a large amount of sunlight onto a small area of photovoltaic cells to generate electricity. This concentration of solar radiation can increase the efficiency of electricity generation, which allows for reduced size and cost of the system, when compared to more conventional photovoltaic systems. A CPV cell primarily uses lenses (e.g. Fresnel lens) to focus the sunlight to a small area. Mainly due to the limited absorption capability for light spectrum, the photovoltaic cell cannot convert 100% of the light energy into electrical energy for output. A significant portion of the un-converted energy becomes heat accumulated in the cell, causing the temperature of the component to rise. This undesirably high temperature can lead to increased internal dark current and decreased conversion efficiency of the cell. In particular, the highly concentrated photovoltaic (HCPV) power generation system makes use of Group III-V semiconductors as the solar cell active materials having a higher energy gap than silicon.
For illustration purposes and as an example, FIG. 1(a) shows a cross section view of a solar cell device structure, which includes a ceramic substrate 11, a circuit layout layer 17 disposed on the ceramic substrate 11, and a layer of solar cell 23 formed on the circuit layout layer 17. The ceramic substrate is utilized as a carrier substrate for the solar cell. When in operation, the ceramic substrate may also function as a heat transfer and/or dissipation member for managing the heat generated. Thus, upon exposure to solar radiation, the solar cell (e.g. a CPV cell) absorbs and converts portion of the solar spectrum into electrical energy, which is transferred by the circuit layout layer 17 into an energy storage unit (e.g. a battery, not shown here). Since a significant amount of heat is generated by the solar cell and the circuit layout layer, the ceramic substrate must have a high thermal conductivity in order to effectively and efficiently transfer the heat either directly or through a heat sink to the ambient air. However, ceramic materials typically have a thermal conductivity lower than 100 W/mK, mostly lower than 50 W/mK.
Therefore, in order to alleviate and solve the heat dissipation problem, a metal-based heat sink 25 (e.g. containing heat dissipation fins 31 and a heat dissipation base plate 27) is placed below the ceramic substrate 11 to enhance its heat dissipation efficiency. The base has at least one primary surface (e.g. 29) to collect heat. Each fin has one or multiple heat-dissipating surfaces (e.g. 19). With a relatively low thermal conductivity, a heat sink must have a large number of thin fins to have sufficient amounts of heat-dissipating surfaces. However, such a design increases the complexity of manufacturing processes and the production cost. It is clear that the functions and performances of prior art solar cells are not satisfactory. In real practice, Cu and Al heat sinks are formed with fins or other structures to increase the surface area of the heat sink, often with air being forced across or through the fins to facilitate heat dissipation of heat to the air. However, there are several major drawbacks or limitations associated with the use of metallic heat sinks. One drawback relates to the relatively low thermal conductivity of a metal (<400 W/mK for Cu and 80-200 W/mK for Al alloy). In addition, the use of copper or aluminum heat sinks can present a problem because of the weight of the metal. For instance, pure copper weighs 8.96 grams per cubic centimeter (g/cm3) and pure aluminum weighs 2.70 g/cm3. If metallic heat sinks are employed, the sheer weight of the metal on the CPC support system can increase the chances of structural failure or other undesirable effects. More significantly, most metals have corrosion issues in an outdoor environment. Further, many metals do not exhibit a high surface thermal emissivity and thus do not effectively dissipate heat through the radiation mechanism.
Thus, there is a strong need for a non-metallic heat sink system effective for dissipating heat produced by a heat source such as a CPV cell. The heat sink system should exhibit a higher thermal conductivity and/or a higher thermal conductivity-to-weight ratio as compared to metallic heat sinks. These heat sinks must also be mass-producible, preferably using a cost-effective process. This processing ease requirement is essential to competing with metallic heat sinks, which can be readily produced in large quantities using scalable techniques such as extrusion, stamping, and die casting. This is more than just a cost issue. Processing ease can also enable the design and construction of more intricate and efficient heat sink geometries.
One group of materials potentially suitable for heat sink applications is the graphitic carbon or graphite. Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube or carbon nano-fiber (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of a few nanometers to a few hundred nanometers. Their longitudinal, hollow structures impart unique mechanical, electrical and chemical properties to the material. The CNT or CNF is a one-dimensional nano carbon or 1-D nano graphite material.
Bulk natural flake graphite is a 3-D graphitic material with each particle being composed of multiple grains (a grain being a graphite single crystal or crystallite) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals. Each grain is composed of multiple graphene planes that are oriented parallel to one another. A graphene plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane). Although all the graphene planes in one grain are parallel to one another, typically the graphene planes in one grain and the graphene planes in an adjacent grain are different in orientation. In other words, the orientations of the various grains in a graphite particle typically differ from one grain to another.
A graphite single crystal (crystallite) per se is anisotropic with a property measured along a direction in the basal plane (crystallographic a- or b-axis direction) being dramatically different than if measured along the crystallographic c-axis direction (thickness direction). For instance, the thermal conductivity of a graphite single crystal can be up to approximately 1,920 W/mK (theoretical) or 1,800 W/mK (experimental) in the basal plane (crystallographic a- and b-axis directions), but that along the crystallographic c-axis direction is less than 10 W/mK. (typically less than 5 W/mK). Consequently, a natural graphite particle composed of multiple grains of different orientations exhibits an average property between these two extremes.
It would be highly desirable in many applications to produce a bulk graphite particle (containing single or multiple grains) having sufficiently large dimensions and having all graphene planes being essentially parallel to one another along one desired direction. For instance, it is highly desirable to have one large-size graphite entity (e.g. a fully integrated or unitary layer of multiple graphene planes) having the c-axis directions of all the graphene planes being substantially parallel to one another and having a sufficiently large length and/or width for a particular application (e.g. >5 cm2 for use as a heat-spreading sheet on a CPU of a smart phone or a 50 cm×50 cm substrate supporting 20 CPV cells). Thus far, it has not been possible to produce this type of large-size unitary graphene entity from existing natural or synthetic graphite particles.
The constituent graphene planes of a graphite crystallite can be exfoliated and extracted or isolated from a graphite crystallite to obtain individual graphene sheets of carbon atoms provided the inter-planar van der Waals forces can be overcome. An isolated, individual graphene sheet of carbon atoms is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction with an inter-graphene plane spacing of 0.3354 nm is commonly referred to as a multi-layer graphene. A multi-layer graphene platelet has up to 300 layers of graphene planes (<100 nm in thickness), but more typically up to 30 graphene planes (<10 nm in thickness), even more typically up to 20 graphene planes (<7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community). Single-layer graphene and multi-layer graphene sheets are collectively called “nano graphene platelets” (NGPs). Graphene sheets/platelets or NGPs are a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted in October 2012; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
NGPs are typically obtained by intercalating natural graphite particles with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 1(c) (process flow chart) and FIG. 1(d) (schematic drawing). The presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter-graphene spacing (d002, as determined by X-ray diffraction), thereby significantly reducing the van der Waals forces that otherwise hold graphene planes together along the c-axis direction. The GIC or GO is most often produced by immersing natural graphite powder (20 in FIGS. 1(c) and 100 in FIG. 1(d)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium perchlorate). The resulting GIC (22 or 102) is actually some type of graphite oxide (GO) particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water. There are two processing routes to follow after this rinsing step:
Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of typically 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid expansion by a factor of 30-300 to form “graphite worms” (24 or 104), which are each a collection of exfoliated, but largely un-separated graphite flakes that remain interconnected. A SEM image of graphite worms is presented in FIG. 2(a).
In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets or foils (26 or 106) that typically have a thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm). Alternatively, one may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite flakes” (108) which contain mostly graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition).
Exfoliated graphite worms, expanded graphite flakes, and the recompressed mass of graphite worms (commonly referred to as flexible graphite sheet or flexible graphite foil) are all 3-D graphitic materials that are fundamentally different and patently distinct from either the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbon material (graphene sheets or platelets, NGPs). Flexible graphite (FG) foils can be used as a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of typically less than 500 W/mK (more typically <300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm. These low conductivity values are a direct result of the many defects, wrinkled or folded graphite flakes, interruptions or gaps between graphite flakes, and non-parallel flakes (e.g. SEM image in FIG. 2(b)). Many flakes are inclined with respect to one another at a very large angle (e.g. mis-orientation of 20-40 degrees).
In Route 1B, the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively called NGPs, 33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm. In the present application, the thickness of multi-layer NGPs is typically less than 20 nm.
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.3354 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 0.001%-10% by weight, more typically 0.01%-5% by weight.
For the purpose of defining the claims of the instant application, NGPs include discrete sheets/platelets of single-layer and multi-layer graphene, graphene oxide, or reduced graphene oxide with an oxygen content of 0-10% by weight, more typically 0-5% by weight, and preferably 0-2% by weight. Pristine graphene has essentially 0% oxygen. Graphene oxide (including RGO) can have 0.001%-46% by weight of oxygen.
The GO molecules in graphene oxide gel, to be described in detail later, typically contains 20-50% by weight oxygen (more typically 30-47%) immediately after removal of the liquid from the GO gel, but prior to a subsequent heat treatment. The GO gel refers to a homogeneous solution of highly hydrophilic aromatic molecules (graphene oxide molecules bearing oxygen-containing groups, such as —OH, —COOH, and >O, on molecular planes or at the edges) dissolved (not just dispersed) in a liquid (e.g. acidic water). The GO gel per se does not contain visibly discernible or discrete graphene or GO particles in the form of solid sheets or platelets. These GO molecules and the dispersing liquid medium have comparable indices of refraction, making the resulting gel optically transparent or translucent, showing lightly brown color. In contrast, the simple mixture of original graphite particles or discrete NGP sheets/platelets with acids and/or water appears optically dark and opaque. These particles or NGP platelets are simply dispersed (not dissolved) in the fluid medium.
These GO molecules in a GO gel are highly reactive and may be considered as “living giant molecules”. By contrast, the prior art solid sheets/platelets of graphene, GO, and RGO are essentially “dead” species. The GO gel can be formed into a shape with a proper shearing or compression stress (e.g. via casting or molding), dried (with liquid components partially or totally removed), and heat-treated under certain conditions to obtain a unitary graphene material, which is typically a single crystal, a poly-crystal with incomplete or poorly delineated grain boundaries, or a poly-crystal with very large grain sizes. The heat treatment serves to chemically link these active or live GO molecules to form a 2-D or 3-D network of chemically bonded graphene molecules of essentially infinite molecular weights, and to drastically reduce the oxygen content of GO down to below 10% by weight, more typically <5%, further more typically <2%, and most typically <<1%. Only a trace amount of oxygen (practically 0%) can survive if the heat treatment temperature is sufficiently high and heat treatment time sufficiently long. This new and unique material called “unitary graphene material” will be further described in detail later.
Although the GO gel per se does not contain visibly discernible/discrete graphene sheets/platelets or NGPs (including “dead” GO sheets/platelets), one can intentionally add discrete graphene sheets/platelets, expanded graphite flakes, and other type of solid filler in the GO gel to form a mixture gel. This mixture gel may be dried and subjected to the same heat treatment to convert the live GO molecules into a unitary graphene material, also enabling these active molecules to chemically bond to the filler particles. This graphene oxide gel-derived graphene material, reinforced with a filler phase (e.g. discrete NGPs, CNTs and carbon fibers), constitutes the presently invented unitary graphene matrix composite as a readily mass-processible material for heat sinks or other heat transfer members. It may be noted that GO may be used to deposit a layer on a surface of a flexible graphite foil 35 to produce GO gel-coated graphite foil 36, which can be dried to form GO-coated graphite foil 40. Similarly, GO may be coated to a pristine graphene graphene paper/film 38 to produce GO gel-coated pristine graphene foil, which upon drying becomes GO-coated pristine graphitic foil 42.
It may be noted that flexible graphite foils (obtained by compressing or roll-pressing exfoliated graphite worms) for electronic device thermal management applications (e.g. as a heat sink material) have the following major deficiencies:                (1) As indicated earlier, flexible graphite (FG) foils exhibit a relatively low thermal conductivity, typically <500 W/mK and more typically <300 W/mK. By impregnating the exfoliated graphite with a resin, the resulting composite exhibits an even lower thermal conductivity (typically <<200 W/mK, more typically <100 W/mK).        (2) Flexible graphite foils, without a resin impregnated therein or coated thereon, are of low strength, low rigidity, and poor structural integrity. The high tendency for flexible graphite foils to get torn apart makes them difficult to handle in the process of making a heat sink, or integrating in a solar cell module or system. As a matter of fact, the flexible graphite sheets (typically 50-200 μm thick) are so “flexible” that they are not sufficiently rigid to make a fin component material for a heat sink or to make a solar cell module supporting structure.        (3) Another very subtle, largely ignored or overlooked, but critically important feature of FG foils is their high tendency to get flaky with graphite flakes easily coming off from FG sheet surfaces and emitting out to other parts of a microelectronic device or solar cells. These highly electrically conducting flakes (typically 1-200 μm in lateral dimensions and >100 nm in thickness) can cause internal shorting and failure of electronic devices.        (4) Both resin-free flexible graphite and resin-impregnated FG (with resin impregnating step occurring before or after roll-pressing) are not conducive to mass production of finned heat sink structures. It is virtually impossible to use mass production processes (such as extrusion, stamping, forging, and die casting that are commonly used for making aluminum heat sinks, or injection molding for making conductive filler-reinforced plastic-based heat sinks) to make FG-based heat sinks without some kind of subsequent bonding or assembling operations. One has to manually attach individual fin members to a core or base member. For instance, one may produce bonded fin heat sink assemblies in which each fin in the assembly is individually bonded into a heat sink base. A major shortcoming of such heat sinks is their high cost. This cost is related directly to the labor required to individually arrange each fin on some sort of support or substrate (a base or core) and high production cycle time. Further, bonding between a fin and a base is not always reliable and the long-term reliability of flexible graphite-based finned heat sinks is highly questionable. Flexible graphite based heat sinks (essentially all of them having resin impregnation or resin coating) are disclosed in the following patents: J. Norley, et al., “Graphite-based heat sinks,” U.S. Pat. No. 6,503,626 (Jan. 7, 2003); M. D. Smalc, et al., “Radial finned heat sink,” U.S. Pat. No. 6,538,892 (Mar. 25, 2003); G. Getz, et al., “Heat sinks made from longer and shorter graphite sheets,” U.S. Pat. No. 6,771,502 (Aug. 3, 2004).        
Similarly, solid NGPs (including discrete sheets/platelets of pristine graphene, GO, and GRO), when packed into a film, membrane, or paper sheet (34 or 114) of non-woven aggregates, typically do not exhibit a high thermal conductivity unless these sheets/platelets are closely packed and the film/membrane/paper is ultra-thin (e.g. <1 μm, which is mechanically weak). This is reported in our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin film or paper sheets (<10 μm) are difficult to produce in mass quantities, and difficult to handle when one tries to incorporate these thin films as a heat sink material. In general, a paper-like structure or mat made from platelets of graphene, GO, or RGO (e.g. those paper sheets prepared by vacuum-assisted filtration process) exhibit many defects, wrinkled or folded graphene sheets, interruptions or gaps between platelets, and non-parallel platelets (e.g. SEM image in FIG. 3(b)), leading to relatively poor thermal conductivity, low electric conductivity, and low structural strength. These papers or aggregates of discrete NGP, GO or RGO platelets alone (without a resin binder) also have a tendency to get flaky, emitting conductive particles into air.
Our earlier application (U.S. application Ser. No. 11/784,606) also disclosed a mat, film, or paper of NGPs infiltrated with a metal, glass, ceramic, resin, and CVD carbon matrix material (graphene sheets/platelets being the filler or reinforcement phase, not the matrix phase in this earlier application). Haddon, et al. (US Pub. No. 2010/0140792, Jun. 10, 2010) also reported NGP thin film (mat/paper of discrete graphene platelets) and NGP-reinforced polymer matrix composites for thermal management applications. The processes used by Haddon et al to produce NGPs are identical to those disclosed much earlier by us (Jang, et al. U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004)). The NGP-reinforced polymer matrix composites, as an intended thermal interface material, have very low thermal conductivity, typically <<2 W/mK. The NGP films of Haddon, et al are essentially non-woven aggregates of discrete graphene platelets, identical to those of our earlier invention (U.S. application Ser. No. 11/784,606). Again, these aggregates have a great tendency to have graphite particles flaking and separated from the film surface, creating internal shorting problem for the electronic device containing these aggregates. They also exhibit low thermal conductivity unless made into thin films (10 nm-300 nm, as reported by Haddon, et al) which are very difficult to handle in a real device manufacturing environment. Balandin, et al (US Pub. No. 2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced by CVD deposition or diamond conversion for heat spreader application. More recently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene Nanoplatelet Metal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metal matrix infiltrated NGPs. However, the metal matrix is too heavy and the resulting metal matrix composite does not exhibit a high thermal conductivity. More significantly, all these prior art materials and related processes are not amenable to mass production of finned heat sinks cost-effectively. In fact, there has been no known report on using these materials for firmed heat sink applications.
Another prior art material for thermal management application is the pyrolitic graphite film. The lower portion of FIG. 1(c) illustrates a typical process for producing prior art pyrolitic graphitic films from a polymer. The process begins with carbonizing a polymer film 46 at a carbonization temperature of 400-1,000° C. under a typical pressure of 10-15 Kg/cm2 for 2-10 hours to obtain a carbonized material 48, which is followed by a graphitization treatment at 2,500-3,200° C. under an ultrahigh pressure of 100-300 Kg/cm2 for 1-24 hours to form a graphitic film 50. It is technically utmost challenging to maintain such an ultrahigh pressure at such an ultrahigh temperature. This is a difficult, slow, tedious, energy-intensive, and very expensive process. Furthermore, carbonization of certain polymers (e.g. polyacrylonitrile) involves the emission of toxic species.
A second type of pyrolytic graphite is produced by high temperature decomposition of hydrocarbon gases in vacuum followed by deposition of the carbon atoms to a substrate surface. This vapor phase condensation of cracked hydrocarbons is essentially a chemical vapor deposition (CVD) process. In particular, highly oriented pyrolitic graphite (HOPG) is the material produced by the application of uniaxial pressure on deposited pyrocarbon or pyrolytic graphite at very high temperatures (typically 3,000-3,300° C.). This entails a thermo-mechanical treatment of combined mechanical compression and ultra-high temperature for an extended period of time in a protective atmosphere; a very expensive, energy-intensive, and technically challenging process. The process requires ultra-high temperature equipment (with high vacuum, high pressure, or high compression provision) that is not only very expensive to make but also very expensive and difficult to maintain. Even with such extreme processing conditions, the resulting PG (including HOPG) still possesses many defects, grain boundaries, and mis-orientations (neighboring graphene planes not parallel to each other), resulting in less-than-satisfactory in-plane properties. Typically, the best prepared HOPG sheet or block remains far from being a graphite single crystal; instead, it typically still contains many grains or crystals and a vast amount of grain boundaries and defects. All PG film production processes do not allow for impregnation of a resin matrix. PG or HOPG films, being weak, non-rigid, and not easily processable suffer from the same shortcomings as flexible graphite intended for use to construct finned heat sinks.
Similarly, the most recently reported graphene thin film (<2 nm) prepared by catalytic CVD of hydrocarbon gas (e.g. C2H4) on Ni or Cu surface is not a single-grain crystal, but a poly-crystalline structure with many grain boundaries and defects. With Ni or Cu being the catalyst, carbon atoms obtained via decomposition of hydrocarbon gas molecules at 800-1,000° C. are deposited onto Ni or Cu foil surface to form a sheet of single-layer or few-layer graphene that is poly-crystalline. The grains are typically much smaller than 100 μm in size and, more typically, smaller than 10 μm in size. These graphene thin films, being optically transparent and electrically conducting, are intended for applications such as the touch screen (to replace indium-tin oxide or ITO glass) or semiconductor (to replace silicon, Si). However, these ultra-thin polycrystalline graphene films are not sufficiently thermally conducting (too many grains or too much grain boundaries, and all grains being oriented in different directions) and not sufficiently thick for use as a heat sink material. Furthermore, the Ni- or Cu-catalyzed CVD process does not lend itself to the fabrication of a heat sink that is typically complex in shape.
Thus, it is an object of the present invention to provide a heat dissipation member formed of graphene oxide (GO) gel-derived unitary graphene material (monolithic graphene entity) or its composite version (containing, for instance, a carbon/graphite filler phase dispersed in or bonded by a unitary graphene matrix material derived from a GO gel), which exhibits a thermal conductivity comparable to or greater than the thermal conductivities of the PG (including HOPG), CVD graphene film, and/or flexible graphite (including resin-impregnated FG). This member has at least one surface deposited with or connected to a layer of electrically non-conducting inorganic material (e.g., ceramic, such as silicon nitride, or semiconductor, such as silicon), which preferably has a good thermal conductivity (preferably >30 W/mK and more preferably >50 W/mK). This inorganic material electrically isolates the CPV cell(s) from the unitary graphene member.
This thermally and electrically conductive graphene monolith or graphene matrix composite can be used to produce a heat dissipation member (such as a finned heat sink or a heat transfer plate) cost effectively in large quantities, using commonly used, less complex, and easier-to-control processes with readily available, inexpensive equipment.
It is another object of the present invention to provide a heat dissipation member formed of GO-derived unitary graphene or graphene matrix composite that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, surface hardness, and scratch resistance unmatched by any material. This member preferably has at least one surface deposited with or connected to a layer of electrically non-conducting inorganic material. This member can include a heat sink, wherein the fins and the base (or core) portion of the heat sink are preferably formed into an integral body that does not involve attaching or bonding individual fin components to the base, or stacking and assembling individual fin sheets together (as would be required in assembling flexible graphite-based fumed heat sinks).
It is a specific object of the present invention to provide a highly conductive unitary graphene material or graphene matrix composite that meets the following technical requirements (a) a thermal conductivity greater than 600 W/mK (preferably greater than 1,000 W/mK, and further preferably greater than 1,700 W/mK); (b) an electrical conductivity greater than 2,000 S/cm (preferably >3,000 S/cm, more preferably >5,000 S/cm, even more desirably >10,000 S/cm, and most preferably >15,000 S/cm); (c) Rockwell surface hardness value >60 (preferably >80); and/or (d) a tensile or flexural strength greater than 80 MPa (preferably >100 MPa, more preferably >150 MPa, and most preferably >200 MPa). No prior art material meets this set of technical requirements.
This new class of heat transfer materials (i.e., a GO gel-derived unitary graphene monolithic and the unitary graphene matrix composite) has the following characteristics (separately or in combination) that distinguish themselves from PG, HOPG, CVD graphene film, flexible graphite sheets, flexible graphite composite, paper/film/membrane of discrete graphene/GO/RGO sheets/platelets, and conventional graphene/GO/RGO platelet-reinforced resin matrix composite, metal matrix composite, and carbon matrix composites:
(1) This unitary graphene material is an integrated graphene entity that is either a graphene single crystal (single grain only) or a poly-crystal (multiple grains but typically having incomplete grain boundaries or having exceptionally large grains). Typically and preferably, with some compression or shearing stresses exerted on the GO during shaping and subsequent heat treating, the unitary graphene material has all the graphene planes in all the grains being essentially oriented parallel to one another (i.e., the crystallographic c-axis of all grains pointing in an identical direction).
(2) The unitary graphene matrix is an integrated graphene entity that is not an aggregate or stack of multiple discrete graphite flakes or discrete platelets of graphene/GO/RGO, and does not contain any discernible or discrete flake/platelet derived from the original GO gel.
(3) This integrated graphene matrix is not made by bonding discrete flakes/platelets together with a binder or adhesive. Instead, GO molecules in the GO gel are chemically active and live species capable of chemically merging with one another mainly in an edge-to-edge manner (forming 2-D giant graphene molecules), but possibly also with adjacent GO molecules below or above (forming 3-D network of graphene chains). Through joining or forming of covalent bonds with one another, the GO molecules are adhered into an integrated graphene entity (the unitary graphene material), without using any externally added linker or binder molecules or polymers. In the presence of discrete carbon/graphite/graphene filler particles (e.g. carbon black particles, CNTs, and NGPs), the GO molecules are also capable of acting as a binder or adhesive that chemically bonds these carbon/graphite filler particles together to form a strong composite.
(4) This unitary or monolithic graphene matrix (a single crystal or poly-crystal with essentially all graphene planes having an identical crystallographic c-axis) is derived from a GO gel, which is in turn obtained from heavy oxidation of natural graphite or artificial graphite particles originally having multiple graphite crystallites. Prior to being chemically oxidized to become GO gel, these starting or original graphite crystallites have an initial length (La in the crystallographic a-axis direction), initial width (Lb in the b-axis direction), and thickness (Lc in the c-axis direction). The resulting unitary graphene entity typically has a length or width significantly greater than the La and Lb of the original graphite crystallites.
(5) It may be noted that there has been numerous reports on “graphene composites.” However, these “graphene composites” make use of discrete pristine graphene sheets, graphene oxide platelets, or reduced graphene oxide platelets as the reinforcement phase, which is dispersed in a matrix material selected from a resin (to form a resin matrix composite), a metal (metal matrix composite), a carbon (carbon matrix composite), a glass (glass matrix composite), or a ceramic (ceramic matrix composite). In these prior art “graphene composites,” graphene sheets/platelets are the discrete and dispersed phase, not the matrix phase (i.e., not the continuous phase that bonds and protects the dispersed phase). These discrete graphene sheets/platelets are the dispersed phase bonded and protected by a matrix material, such as a resin, metal, carbon (CVD carbon, amorphous carbon, or polymeric carbon), glass, or ceramic. In stark contrast or completely oppositely, in the presently invented unitary graphene matrix composite, graphene is the matrix material that serves to bond, adhere, and protect the dispersed filler phase, such as CNT and carbon black (CB) particles. CNT or CB particles are dispersed in and protected by the unitary graphene matrix. The graphene matrix is a continuous, unified, or integrated material phase.
The present invention also provides a method or process for producing heat sinks and/or heat transfer plates from such a GO gel-derived unitary graphene entity and the graphene matrix composite.
Another object of the present invention is to provide a cost-effective process for producing heat sinks or heat transfer plates from a GO-derived graphene monolith and a graphene matrix composite that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, surface hardness, and scratch resistance.
In particular, the present invention provides a fast, scalable process capable of mass-producing unitary or monolithic graphene or graphene matrix composite-based heat sinks from a GO gel. Advantageously and surprisingly, conventional mass production techniques, such as die casting, injection molding, compression molding, resin-transfer molding, and extrusion, can be adapted for cost-effectively producing these heat sinks.
This process involves significantly lower heat treatment temperatures as compared with the processes for producing pyrolytic graphite (including HOPG) from either carbonized polymers (e.g. polyimide) or the CVD graphite. The presently invented process is simpler (hence, more reliable), less energy-intensive, and highly scalable.